One previously known way of rotary cutting is to pass a web of material between a pair of superimposed and rotating metal cylinders with one cylinder having a plain cylindrical surface acting as an anvil for cutting elements carried by the other cylinder. The cutting elements project generally radially outwardly from the body of the other cylinder and have a sharp knife edge with a generally V-shaped cross-section. The cutting elements can be either separate inserts received in the cylinder or a homogenously integral part of the cylinder. A method and apparatus for producing the V-shaped cutting elements as an integral part of a metal cylinder is disclosed in U.S. Pat. Nos. 3,550,479 and 3,796,851.
These rotary dies will satisfactorily cut paper, paper board, metal foil and other materials having a relatively high density. With such relatively high density materials, once the sharp knife-edge cuts through about 50% to 60% of their thickness, they are compressed sufficiently to burst or separate with their cut edges having a relatively clean and smoothly cut appearance to the naked eye.
However, such rotary dies are impractical for cutting sheets of low density materials such as films and non-woven fibers which usually have a thickness of about 0.005 to 0.030 of an inch. Such non-woven fiber materials typically have an individual fiber thickness of less than about 0.0002 of an inch. Typical low density materials are polyester, polypropylene and polyethylene plastics. When cutting a low density material, the sharp knife-edge must pass through essentially the entire thickness of the material to sever it because it flows easily and does not have a tendency to burst, unlike a high density material. To produce an edge having a smooth and clean cut appearance to the naked eye, it is also necessary for the sharp knife-edge to pass essentially completely through the thickness of a low density material to avoid stringers and jagged edges.
In order to satisfactorily cut or sever a web of a low density material with rotary cutting dies on a mass production basis, it is necessary to orient and position the rotating die cylinders so that between the rotating cutting blade and the anvil there is either a slight minimum clearance, no clearance, or a slight compression or interference of about .+-.0.0001 of an inch. This is usually referred to as positive, zero or negative clearance. PG,4